This is my most used tool. I think it’s traditionally used for tile work. It’s a square piece of plastic that has different designs on each edge. One is smooth and the other three have different spacing notches. I use it to apply, smooth and scrape paint. It's wonderful at applying the paint without leaving brush marks. Tube Wringer
This little tool is great for getting every last drop of paint out of a tube. You pay a lot for that paint. Don’t throw it out because you can’t get it out. They make a plastic one, but I use the heavy duty metal one. It will be the only one you need to buy, because it will last forever.
These little bottles are great for creating thin continuous lines. The tips are closed so you have to cut them to get the paint out. There are also bottles with metal tips. You can get an extremely thin line using these. The secret to using these is that you need to thin the paint so it will come out. I use Liquitex Fluid Medium. Try to limit mixing your acrylic paints with water. Using water will weaken the paint and it may flake right off the canvas.
I love these tools. They are like a brush only they don’t have a brush head. It’s a flexible silicone. They are just so different. Almost like a soft palette knife. Think baking spatula. These are amazing at applying paint in a smooth finish. They are also useful for mixing paint.
Camera (in my case, a cell phone camera)
I know this is a little unusual for a tool, but I find that I use it for every painting. Taking in-progress pictures helps show the evolution of a piece. Looking at the picture on a computer provides a different perspective. I can easily catch things that need to be changed. Reviewing the photos makes any design problems more obvious. I think of it as the modern day mirror. And after you've finished the painting, you have an image ready to post on Etsy so you can start making sales.